Thrombopoietin (TPO), also called megakaryocyte growth and development factor (MGDF), thrombocytopoiesis stimulating factor (TSF), c-myeloproliferative leukemia ligand (c-Mpl), mpl ligand, or megapoietin, is a glycoprotein that has been shown to be involved in the production of platelets. See Wendling, F., et. al., Biotherapy 10(4): 269-77 (1998); Kuter D. I. et al., The Oncologist, 1: 98-106 (1996); Metcalf, Nature 369: 519-520 (1994).
Under certain circumstances, the activity of TPO results from the binding of TPO with the TPO receptor (also called MPL). The TPO receptor has been cloned and its amino acid sequence has been described. See Vigon et al., Proc. Nat. Acad. Sci., 89: 5640-5644 (1992).
TPO is a 332-amino acid glycosylated polypeptide that plays a key role in the regulation of megakaryocytopoiesis, and in the process in which platelets are produced by bone marrow megakaryocytes. See Kuter et al., Proc. Natl. Acad. Sci. USA 91: 11104-11108 (1994); Barley et al., Cell 77:1117-1124 (1994); Kaushansky et al., Nature 369:568-571 (1994); Wendling et al., Nature 369: 571-574 (1994); and Sauvage et al., Nature 369: 533-538 (1994). TPO is produced in the liver but functions mainly in the bone marrow, where it stimulates the differentiation of stem cells into megakaryocyte progenitors, and stimulates megakaryocyte proliferation, polyploidization and, ultimately, enters the platelet circulation in the body. TPO is also a primary regulator in situations involving thrombocytopenia and in a number of studies that include increasing platelet counts, platelet size and isotope incorporation into platelets of recipient animals. See, Metcalf Nature 369: 519-520 (1994). Specifically, TPO is considered to affect megakaryocytopoiesis by several ways: (1) it causes increase in megakaryocyte size and number; (2) it increases DNA contents, the forms of polyploidy, and the number of megakaryocytes; (3) it increases megakaryocyte endomitosis; (4) it increases the number of mature megakaryocytes; (5) it increases the percentage of precursor cells, the number of small acetylcholinesterase positive cells, the number of bone marrow cells.
Platelets are necessary for blood clotting. When platelet numbers are very low, a patient is at risk of death from catastrophic hemorrhage. Thus, TPO has been used for both the diagnosis and the treatment of various hematological disorders, for example, diseases primarily caused by platelet defects. Likewise, TPO may be useful for the treatment of thrombocytopenic conditions, especially those derived from chemotherapy, radiation therapy, or bone marrow transplantation for the treatment of cancer or lymphoma.
Because the slow recovery of platelet levels in patients suffering from thrombocytopenia is a serious problem, it would be desirable to provide a compound for the treatment of thrombocytopenia by acting as a TPO mimetic. A few years ago, the development of TPO peptide mimetics was reported (WO96/40750, WO98/25965). These peptides were designed to bind and activate the TPO receptor (TPO-R) but have no sequence homology to the natural TPO. In recent years, a number of active small-molecule TPO mimetics have been reported, including 1,4-benzodiazepin-2-ones (JP11001477), metal complexes derived from Schiff base ligands (WO99/11262), cyclic polyamine derivatives (WO00/28987), thiazol-2-yl-benzamides (WO01/07423, WO01/53267), azo-aryl derivatives (WO00/35446, WO01/17349), 2-aryl-naphthimidazoles (WO01/39773, WO01/53267), and semicarbazone derivatives (WO01/34585). In cell-based systems, all of these molecules can activate signal transduction pathways that are dependent on the presence of the TPO receptor on the cell membrane. Certain types of compounds can directly act on the TPO receptor itself. It has been reported that certain substituted thiosemicarbazone derivatives are actually effective agonists of the TPO receptor. Some of the most preferred compounds of this series were found to stimulate the proliferation and differentiation of TPO-responsive human cell lines and TPO in human bone marrow cultures that has a concentration below 100 nM.
Several patents assigned on their face to GlaxoSmithKline describe a thrombopoietin analog, eltrombopag (WO00/189457, WO01/089457, WO2006/064957), which shows good activity.